Formation of a color photographic image requires recording the blue, green, and red content of the scene in spectrally sensitized emulsions. The three sensitive emulsions are most commonly coated in a multilayer arrangement. All silver halide emulsions have natural sensitivity to blue and ultraviolet radiation. Emulsions intended for the magenta layer(s) are dyed with a green-light-absorbing sensitizing dye and have green sensitivity in addition to the natural blue sensitivity; those intended for the cyan layer(s) are dyed with a red light absorbing sensitizing dye and have red sensitivity in addition to the natural blue sensitivity. The usual layer order of a multilayer color film places the blue-sensitive (yellow dye forming) layer(s) on the top where they are the first of the sensitive layers to be exposed by the incident light. A yellow filter layer is usually coated below the blue sensitive layer to absorb any residual blue light and to prevent the green-sensitive (magenta dye forming) and red sensitive (cyan dye forming) layers, coated beneath, from being exposed by blue light. Such exposure of the green-sensitive and red-sensitive layers by blue light is undesirable because it would result in degradation of colors. There is a disadvantage to this layer arrangement commonly used in multilayer color films, because of the scatter of light by silver halide grains. The green and red light must pass through the blue-sensitive emulsion layer and the yellow filter layer before it can expose the green- and red-sensitive emulsion layers. The scattering of the green and red light by the silver halide and the material making up the yellow filter layer results in a degradation or distortion of the images formed in the green- and red-sensitive emulsions layers. Since the human eye is most sensitive to green light, the degradation of the image in the green-sensitive layer is of greatest consequence. Among the potential solutions to this problem would be to coat the green-sensitive layer in the uppermost position where the incident light entering the film would strike it first.
Multilayer color films with the green-sensitive emulsion layer and the red-sensitive emulsion layer coated above the blue-sensitive layer--so-called "inverted structure" films--are known in the art. Such films have not been fully satisfactory, however, as to speed and image structure. This is because in the inverted order films the blue-sensitive layer, being on the bottom of the film, was required to be of very high speed because much of the blue light entering the film was absorbed by masking couplers for camera speed materials and absorbed and scattered by the green-sensitive and red-sensitive emulsion layers before reaching the blue sensitive emulsion layer. This problem is alleviated by using emulsions in the green-sensitive and red-sensitive layers which are high in chloride and have minimal absorption in the blue region of the electromagnetic spectrum. Such emulsions, however, are less suitable for attaining speeds adequate for camera speed application and their use has been restricted to materials intended for producing prints from color negatives (e.g. Eastman Color Print Film).
Another problem in such inverted structure films is the minus blue/blue exposure separation which is assessed by measuring the difference between the speed of the blue sensitive layer and the speed of the green-sensitive or the red-sensitive layer when the film is exposed to blue light. In films possessing the usual layer order, the speed to a blue light exposure of the green-sensitive and red-sensitive layers is at least 1.0 log exposure units less than the speed of the blue-sensitive layer. This lower speed is primarily due to the absorption of blue light by the masking couplers, blue-sensitive layer, and a blue light filter such as a yellow dye or yellow colloidal silver. In the inverted structure films, blue light reaches the green-sensitive and/or red-sensitive layers without being attenuated by such blue-sensitive and yellow filter layers. This results in more exposure by blue light and less minus blue/blue exposure separation for the green- and red-sensitive layers.
U.S. Pat. Nos. 4,439,520 and 4,672,027 disclose the use of tabular grain emulsions of high aspect ratio to improve the minus blue/blue exposure separation when used in multicolor photographic elements and recommends their use in the green-sensitive and red-sensitive layers of films in which these layers are coated above the blue-sensitive layer. This provides an improvement in the art but the loss in blue light to absorption and scatter by the overlying imaging layers requires the use of the highest speed blue-sensitive emulsions in the underlying blue sensitive layer and such emulsions have a high level of graininess and limit the maximum speed attainable by the multilayer color film.
Thus, although it is possible to produce multilayer color films in which the green-sensitive and red-sensitive layers are coated above the blue sensitive layer, and these films have the superior sharpness associated with the elimination of scatter of the green light and minimizing that of the red light, there is a limitation in the maximum speed attainable because such films have a high level of yellow graininess and the minus-blue/blue exposure is barely acceptable. Further improvements in inverted structure films to take advantage of the enhanced sharpness but with improved yellow layer graininess, better minus-blue/blue exposure separation and at higher speed are highly desirable. Better minus-blue/blue exposure means that there is a greater difference in the speed of a green-sensitive or red-sensitive layer when exposed to light not containing blue light than when exposed to blue light than a comparative film emulsion.